Conventionally, a variety of catalysts for purification of exhaust gas have been developed to purify harmful substances emitted from internal combustion engines of automobiles and the like. A catalyst with a honeycomb filter as a base material made to support a metal oxide and a noble metal is generally used as a catalyst for emission gas purification. Examples of such a metal oxide are alumina, zirconia, and ceria, while examples of such a noble metal are platinum, rhodium, and palladium.
As examples of such a metal oxide, ones in powder obtained by wet-milling are conventionally used. For example, Japanese Unexamined Patent Application Publication No. Hei 10-182155 (Document 1) and Japanese Unexamined Patent Application Publication No. 2002-79097 (Document 2) disclose a method of obtaining the powder of a composite metal oxide. In the method, an oxide precursor, having been prepared from a salt solution of aluminum, cerium, or zirconium by the coprecipitation method, is calcined in the air to obtain the powder of the composite metal oxide. In addition, Japanese Unexamined Patent Application Publication No. Hei 7-300315 (Document 3) discloses a method of obtaining the powder of a composite metal oxide. In the method, the powder of boehmite alumina is added to a mixture solution of a cerium salt aqueous solution and a zirconium salt aqueous solution, and stirred. The resultant solution is then dried and calcined to obtain the powder of the composite metal oxide.
In such cases described in the above documents 1 to 3, however, use of a conventional powder of metal oxide causes the following problem. When a base material such as a metal honeycomb filter, and a high-density honeycomb which has narrower tubular passages (for example, a micro-channel of 1200 cell/inch2 or more) than that of an ordinary honeycomb filter is used, it is difficult for the base material to support such a conventional powder of metal oxide. This is because the adhesion of such a conventional powder of metal oxide to the base material (especially to metal base material) is not necessarily strong enough yet. This is also because the heat resistance thereof is not necessarily high enough yet. In addition, there is a limit in forming a thin-film coating on a base material using the conventional metal oxide powder.